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A study of radio continuum emission of the Milky Way Galaxy

Abstract

The synchrotron emissivity distribution of the Milky Way Galaxy has been modelled from the 408 MHz allsky survey of Haslam et al.(1982) after separation of its thermal component with the help of IRAS 60 micron emission(Broadbent et al 1989). We have refined the spiral arm pattern in the inner part of the Galaxy by including a bar at the Galactic centre and an updated the Galactic distance scale and obtained fitted free parameters. At 408 MHz, there is very little absorption in the interstellar medium and the line of sight distribution of synchrotron emissivity was inferred mainly from its presumed relationship to the other tracers of spiral structure via these fitted free parameters. At lower frequencies, the absorption of synchrotron emission due to thermal electrons becomes significant and can give direct information of the nonthermal distribution along the line of sight. We have modelled the distribution of thermal electrons according to our synchrotron arm model and an alternative model based on pulsar dispersion measures using the Galactic rotation curve and the surveys of the distribution of Hl66a emission We have then used our synchrotron model applied at lower frequencies including the absorption to compare with the surveys of Dwarakanath et al.(l990) at 34.5 MHz and Jones and Finlay(l974) at 29.9 MHz. The result confirms that the absorption model of the synchrotron emissivity in the Galactic plane is broadly correct and illustrates the potentials of the absorption technique. However we were not able to distinguish the two models of ionised hydrogen spiral structure. To do this, recombination line surveys with improved frequency resolution are required. Using a new value of the cosmic ray gradient in the Galaxy from diffuse gamma-ray emission we obtain the separate variation of magnetic field and cosmic ray electron density. We give the global properties of the thermal and nonthermal emission that our model implies.